The present invention relates to the field of fuel injection in turbomachines, and it relates more particularly to a fuel metering unit having two outlets (also known as a dual port unit).
In turbomachines, the best performance is obtained when the rotary members are rotating at speeds close to the maximum acceptable limit beyond which there is a danger of losing compressor or turbine blades. Such blade loss is liable to puncture the engine casing with consequences that can go so far as complete destruction of the turbomachine.
Thus, and indeed as recommended by international regulations concerning civil aviation, the use of an overspeed limiter device turns out to be essential to ensure that the speed of these rotary members does not exceed a maximum authorized speed.
Furthermore, for present turbomachines, it must be possible for the pilot of an aircraft to return to a normal operating mode after the overspeed limiter has acted. For this purpose, and as shown in FIG. 6, the overspeed valve 10 of the limiter placed at the outlet from the metering unit 12 must be capable of shutting off only the delivery 14 of fuel at a high flow rate without altering (disturbing) the delivery 16 of fuel at low flow rates and supplying, in particular, the xe2x80x9coverspeedxe2x80x9d flow rate to which the metering unit feeding the injectors is restricted once the overspeed valve has indeed closed.
An elegant solution to this problem thus consists in implementing a fuel metering unit having two metered fuel outlets. Such a dual-port unit is described, for example, in U.S. Pat. No. 5,771,182. Unfortunately, the linear structure of that unit, also referred to as xe2x80x9ctandemxe2x80x9d structure, since the metering slots are placed in series one after another, suffers from the severe drawback of lengthening the metering unit very significantly (it is practically twice as long as a conventional unit) and consequently increasing the on-board mass in non-negligible manner.
The present invention thus seeks to mitigate those drawbacks with a fuel metering unit that is particularly compact, i.e. of minimum size and mass. Another object of the invention is also to propose a unit which does not disturb the metered flow rate below the limited overspeed rate.
These objects are achieved by a fuel metering unit in a turbomachine, the unit comprising an outer cylindrical sleeve having mounted therein a distributor cylinder in which a metering piston can move linearly under the action of a control element, said cylindrical sleeve having an inlet orifice for admitting fuel under pressure and at least one outlet orifice for injecting said fuel into a combustion chamber of said turbomachine, and said metering piston including an annular gap forming a distribution chamber for the fuel admitted via said inlet orifice and delivered via said at least one outlet orifice, wherein said distributor cylinder has a plurality of radial feed orifices for admitting fuel into said distribution chamber from said inlet orifice, and both at least one first metering orifice for expelling the fuel to said first outlet orifice, and at least one second metering orifice for expelling the fuel towards said second outlet orifice, said at least one first and at least one second metering orifices being separated by sealing means placed at the periphery of said distributor cylinder.
With the present metering unit structure that includes a leakproof separating partition, it is possible within a single unit to combine supplying fuel both at high flow rates and at low flow rates. The low flow rate circuit is thus not disturbed in any way by the overspeed valve and the size of the unit is particularly small compared with prior art units having two metering units placed one after the other.
In an advantageous embodiment, said first metering orifices are two in number and they are placed diametrically opposite each other. Preferably, said first metering orifices present a flow section that is exponential. Similarly, and advantageously, said second metering orifices are two in number and they are disposed diametrically opposite each other. Preferably, said second metering orifices present a flow section that is exponential.
Said at least one first and at least one second metering orifices are placed in a common transverse plane and they are offset in pairs by 90xc2x0, so as to ensure fuel flow continuity towards said combustion chamber during displacement of said metering piston. Said first metering orifices present a width a at their downstream ends that is equal to a width b of said second metering orifices at their upstream ends.
In a first embodiment, said sealing means comprise an elastomer gasket mounted in a groove of a separating partition extending from the outer periphery of said distributor cylinder and separating said first metering orifice(s) from said second metering orifice(s). Alternatively, said sealing means comprise an elastomer gasket mounted in a groove of a separation well disposed around each of said metering orifices so as to isolate them from said second metering orifices.